Electroplating bath for silver



Patented May 4; 1948 1 2,440,672 ntuc zfaormi me BATH FOR SILVER.

green; Indiana!! lis, 1nd, "assfg' nonto Motors (lorporation, Detroit, Mich; "a t c'oi iioratioir oimeiaware momm Qpfilication October 24, 1942,

. 1 serial No. 463,250

V "4 dlaims.

i liisinveritioh has to dWith-th'e lec'trddepositidn-df metals and 'has Jpa'iti'cul'ai "application toitheelectrodeposition or silver. I tl heiprimary object of I y inviit'fon' is to'piovide a new -an'd improved' 'ba-th and .io'focess "for V1 -the =electrodeposition ot a -nietal such as silver I in-the =form oif' 'bright 'plate. Other objects and -advantages of theinv-erition ivill'become mere apparentirom the mofe detailed descriptionwliich follows; p

invention is basdbn the ius'e of certain addition agents in novlrlatidnship. 'Tl-iebath or electrolyte of the invention is one 'offgood efiine and-is one that is easy-tooperateandcon- --trol.- Theallowable-ranges of current densities, voltagestrang-es-of the constituents-making up the --bath and -other variables for good deposits are quite broad."

"In carrying outtheinven-tion an-addition agent is iprovided for the plating bath which 'inay be termed a brightening and-structure-controlling agent. When a'dded to:plating-bathssuch-as 'the silver-cyanide bath the --si-1ver is Y deposited in the 'form' of'bright' plate.

The "amount of thbrightener-additionagent is relatively small. Successful rsults'--have"been obtained with' concentrations -from 0.05 to 1 =12.5 *unniter 6f plafii solution. The brightener addition a e'n t is used ii-p er is depleted rear-mg.

operationof the bath and mustbe; plemsheu.

The brightener "ad'di-tionagents m a'cc'orrlaiice withthe present ini'r'ention are reaction products of ammonium-thiosulfate with various suitable modifying a'grits. "flfcomibdsitfdnthat has proven highly-satisfactory is a eonime'r'c'ial 'X-ray "fixer used to dissolve silver salts from- X rayfilm after exposure and development. This solution, hereinafter termed solution A, is composed substanbrig-htener; contains in aqueous ;.sohition,w;am-

mon'iufn thiosulfate (60%) -%00 nil.;sodium-suliite 7.5 grams, and acetic acid (28%), 28.5 ml. The solution has a. specific gravity of 1.311 and a pH of 4.12 at 25 C.

Other acids such as formic, hydrochloric, sulfamic, perchloric, sulfuric are possible for use with the thiosulfate-sulfite mixture but the compounds formed are unstable. However, if bisulfites are added to the thiosulfate, benzoic, tartaric, citric, or hydrocyanic acids will produce efiective brightening agents. Acetic and sulfurous acids may also be added to the thiosulfate-bisulfite combination to produce an effective brightener.

32.2, made by adding'246 ml. dfisiilfiirous acid (7.5 i to 100om1. df anufioriium'tfiiosuliate (60%).

Other entitl ment "ingtag''iits which may be added to ammoniu'm lthiostflfate'toiproduce eiiective brighteners are iodine, 'liy'pohlorites, or hydrogen peroxide. Iodine, sodium hypochlorite or hydrogenperoiiidegfiive very good re'sults when added to: a rmomumthibsiuiate. bii't tli li'prbdli'cts are not stable. In general themoiii'fying agents for the ammonium thios'ulfate maybe" termed oxidizing agents.

The following areillufstrative'eiiamplesof plating baths in tvhichlthe brightefieraii'dition has been used.

cyanide is increased.

:AgCN,-'100-12-0. gr/liter of aqueous solution. KCN =t'free) 120-1-50 rg./liter of aqueous :solution.

KOH (free), 15-50 g./liter of aqueous solution. Solution-A; 0105-1225 mh/lit-er of-solution.

Silver cyanide concentrations 'as "high' as 184 gQ/liter have been *successfuny useu experimentally.

Equivalent amouhts'fof the o'ther described lb'righten'er addition agent's -"may'ib'eused "in the Ybaths of EiiamplesIandii inpiace'of solution A.

The bath temperature may" vary'rrom about "ener concentrations" O i5- lf'niliiliter of "plating 's'olution) "for'maximum brilliance ofiieposits. It

I 1 v H H I ishighlydesirableto contrcltempe'ratureswithin Another solution whihmay befused as the :i j5-10 F. and .prf'erabiy'somewhat above room gtemperature-s. At present happens-that the best 'ia'n'ge "of temperatures for; generali work at "current densities of 30-"75"ampsjsfffcathode area is about 90-130 F.

The current density may be increased greatly by using adequate agitation. Other factors extending the current density range are:

Current densities as high as 150 a./s. f. have been obtained with Example I containing 50 g./liter Another efiective brightener solution is one free KOH with effective solution agitation.

The following are illustrative examples of types of agitation which may be employed I. Oscillating rod agitation, 18-50 feet per minute. Temperature, 550-100 F. Current density, 20-40 a./s. f. Brightener, 0.1-0.5 m1./liter of solution. II.

anode stationary.

a. Plate deposited on inside of cylinder (anode within cylinder).

Surface velocity unknown.

Temperature, 80-120 Current density, 20-35 a./s. i.

Brightener, 0.25-1.5 ml./liter.

1). Plate deposited on outside diameter of cylinder (anode outside cylinder).

Surface velocity, 20-400 feet/min.

Temperature, 80-130 F.

Current density, 20-75 a./s. f. (or more). Brightener, 0.25-2 ml./liter of solution. Cylindrical cathode stationary-anode revolving within it. Plate deposited on inside diameter of cathode. Rotation speed, 150-400 R. P. M. Temperature, 80-130 F. Current density, 20-75 a./s. f. Brightener, 0.25-2 ml./liter of solution. 1V. Anode and cylindrical cathode stationary.

Solution moving between anode and cathode, parallel to length of cylinder. Surface velocity, 20-300 feet/minute. Current density, 20-100 a./s. f. (or more). Brightener,0.5-2 mL/Iiter.

Combinations of the above methods of agitation are also used. In every case best results are obtained with the greatest speed of exposure of the cathode surface to fresh solution without excessive turbulence. In cases 11a and III above, an increased speed of rotation does not necessarily imply a greater exposure of cathode to fresh Solution.

Bath voltages vary from 1.2 to 2.8 volts, ordinarily, but depending on current density, distance of anode from cathode, amount of brightener, size of anode and cathode, and composition of bath, may range from 0.8-5 volts.

The brightener addition agent is used up or is depleted during operation of the plating bath. As this occurs more brightener has to be added to maintain the standard of brightness. In one 1200 liter tank 5-20 ml. of brightener are added hourly, This represents about 0.1 to 0.4 ml./liter III.

each 24 hours. If it is wanted to make fewer additions it is possible to add from 0.05 ml. to 4 ml. of brightener per liter of solution, the quantity depending on conditions, and make no further additions until the appearance of the deposits indicates that more is needed. In-this case maximum eifect of the brightener is attained in about six hours, good results being obtained from the Cylindrical cathode revolving in solution- 4 brightened solution for from 8 to 36 hours, depending upon the amount added, current density, the temperature, and other factors. It is best to add no more brightener than is necessary to give the desired results.

By the use of the bath and process of the inventionit is possible to produce in a relatively short time thickdeposits of silver in bright form. Current densities as high as 600 amperes per square foot have been used with bath tempera- .tures up to 165 F. Silver has been plated as high as .050" an hour with good agitation,

I claim:

7 1. A silver cyanide plating bath containing free cyanide and about 0.05 to 12.5 ml. per liter of a solution composed substantially as follows: 32.5% ammonium thiosulfate and ammonium polythionate, 3.5% acetic acid, 3.5% boric acid, 4.2% sodium sulfite, and the remainder water.

2. A silver cyanide plating bath containing free cyanide and to which has been added a brightener addition agent made up substantially as follows: ammonium thiosulfate (60%), 200 ml., an alkali metal sulfite 7.5 grams, and acetic acid (28%), 28.5 ml., said brightener addition agent being present in the platin bath in the proportion within the range .05 to 12.5 ml. per liter of plating solution.

3. A silver-cyanide plating bath comprising es- 1 and a pH of approximately 4.12 to 4.25 at 25 C.

4. A silver-cyanide plating bath comprising essentially an aqueous solutionof silver cyanide and an alkali metal cyanide to which has been added 0.05 to 12.5 ml. per liter'of a brightener comprising essentially an aqueoussolution of ammonium thiosulfate, an alkali metal sulfite and acetic acid, said brightener having a specific gravity of approximately 1.305 to 1.311 and. a pH of approximately 4.12 to 4.25 at 25 C.

V HARRY IJ. GREEN.

REFERENCES CITED,

The following references are of record in the file of this patent? UNITED STATES PATENTS Number Roscoe and Schorlemmer. 

